Heretofore, there have been widely used step-down DC/DC converters in power supply circuits for electronic devices or the like, for turning on and off semiconductor switching elements to convert a DC input power supply voltage into a DC output voltage which is lower than the DC input power supply voltage.
FIG. 1 is a circuit diagram of the arrangement of a conventional step-down DC/DC converter.
As shown in FIG. 1, the conventional step-down DC/DC converter comprises a DC power supply 101, a main transistor 102 as a main switching element having a collector terminal connected to the positive terminal of the DC power supply 101, a main recirculation diode 103 as a main recirculation rectifying element connected between the emitter terminal of the main transistor 102 and the negative terminal of the DC power supply 101, with the emitter terminal of the main transistor 102 being connected to the cathode of the main recirculation diode 103, a series-connected circuit comprising a smoothing reactor 104 and a smoothing capacitor 105 which are connected parallel to the main recirculation diode 103, a load 106 connected parallel to the smoothing capacitor 105, and a control circuit 107 for outputting an on and off control signal to the base terminal of the main transistor 102.
In the step-down DC/DC converter of the above arrangement, depending on a change in the voltage across the smoothing capacitor 105, which is a low DC output voltage, the duration of the on and off control signal applied to the base terminal of the main transistor 102 varies for thereby controlling the on period of the main transistor 102 to stabilize the voltage across the smoothing capacitor 105, i.e., the voltage supplied to the load 106.
FIG. 2 is a diagram showing an overlap between a switching voltage waveform and a switching current waveform of the step-down DC/DC converter shown in FIG. 1.
As shown in FIG. 2, the step-down DC/DC converter shown in FIG. 1 has a problem in that when the main transistor 102 is turned on or off, a collector-to-emitter voltage waveform V.sub.CE of the main transistor 102 and a collector current waveform Ic of the main transistor 102 overlap each other in areas W, and a large switching loss occurs in the overlapping areas W. Another problem is that spike surge voltage V.sub.SR, surge current I.sub.SR, and noise are produced at positive-going edges of the collector-to-emitter voltage waveform V.sub.CE and the collector current waveform I.sub.C.
Japanese laid-open patent publication No. 7-241071 discloses a step-down DC/DC converter capable of reducing the switching loss, the surge voltage, the surge current, etc. as described above.
FIG. 3 is a circuit diagram showing the arrangement of the step-down DC/DC converter disclosed in Japanese laid-open patent publication No. 7-241071.
As shown in FIG. 3, the step-down DC/DC converter disclosed in Japanese laid-open patent publication No. 7-241071 includes, as parts added to the step-down DC/DC converter shown in FIG. 1, a series-connected circuit of an auxiliary transistor 109 as an auxiliary switching element and a resonant reactor 110 which are connected parallel to the main transistor 102, a series-connected circuit of auxiliary recirculation diodes 111, 112 connected between the junction between the auxiliary transistor 109 and the resonant reactor 110 and the negative terminal of the DC power supply 101, a resonant capacitor 108 connected between the junction between the auxiliary recirculation diodes 111, 112 and the emitter terminal of the main transistor 102, and a circulating current diode 113 connected parallel to the main transistor 102.
Operation of the step-down DC/DC converter of the above arrangement will be described below.
When the main transistor 102 is turned off while a current is flowing to the load 106 and the resonant capacitor 108 is being charged up to the power supply voltage in an on state of the main transistor 102, the current that has flowed through the main transistor 102 immediately switches to the current flowing through the resonant capacitor 108, which is then gradually discharged. At this time, the voltage across the main transistor 102 gradually rises from 0 V. Since zero-voltage switching is achieved when the main transistor 102 is turned off, a switching loss caused when the main transistor 102 is turned off can be reduced.
When the control circuit 107 applies an auxiliary control pulse to a control terminal of the auxiliary transistor 109 to turn on the auxiliary transistor 109 before applying a main control pulse to a control terminal of the main transistor 102 to turn on the main transistor 102, the power supply voltage is applied to the resonant reactor 110 and the current flowing through the resonant reactor 110 linearly increases during the period in which the main recirculation diode 103 is conducting. Because zero-voltage switching is achieved when the auxiliary transistor 109 is turned on, a switching loss caused when the auxiliary transistor 109 is turned on can be reduced.
As the current flowing through the resonant reactor 110 increases, the current flowing through the main recirculation diode 103 linearly decreases. When the current flowing through the resonant reactor 110 becomes equal to the load current, the main recirculation diode 103 is cut off. When the main transistor 102 is turned on at this time, the collector-to-emitter voltage of the main transistor 102 immediately drops to 0 V. Inasmuch zero-voltage switching is achieved when the main transistor 102 is turned on, a switching loss caused when the main transistor 102 is turned on can be reduced.
When the auxiliary transistor 109 is subsequently turned off with a slight delay, a resonant current flows through the resonant reactor 110 and the resonant capacitor 108, and the voltage across the resonant capacitor 108 rises from 0 V in a sinusoidal fashion. When the voltage across the resonant capacitor 108 reaches a maximum level, the resonant current becomes nil. When the auxiliary transistor 109 is turned off, a current flows to the smoothing reactor 104 via the main transistor 102. Since zero-voltage switching is achieved when the auxiliary transistor 109 is turned off, a switching loss caused when the auxiliary transistor 109 is turned off can be reduced.
In the manner described above, the switching loss when the main transistor 102 and the auxiliary transistor 109 are turned on and off can be reduced.
Spike surge voltage and surge current generated when the main transistor 102 and the auxiliary transistor 109 are turned on and off are absorbed by the resonant capacitor 108 and the resonant reactor 110. Therefore, a surge voltage, a surge current, and noise can be reduced when main transistor 102 is turned on and off.
FIG. 4 is a circuit diagram showing the arrangement of another conventional step-down DC/DC converter.
As shown in FIG. 4, the conventional step-down DC/DC converter comprises a DC power supply 201, a MOSFET 202 as a main switching element having a drain terminal connected to the positive terminal of the DC power supply 201, a main recirculation diode 203 as a main recirculation rectifying element connected between the source terminal of the MOSFET 202 and the negative terminal of the DC power supply 201, with the source terminal of the MOSFET 202 being connected to the cathode of the main recirculation diode 203, a series-connected circuit of an inductive element 204 and a capacitor 205 which are connected parallel to the main recirculation diode 203, a load 206 connected parallel to the capacitor 205, a resistor 212 having a terminal connected to the positive terminal of the DC power supply 201, a capacitor 208 having a terminal connected to the other terminal of the resistor 212 and another terminal connected to the source terminal of the MOSFET 202, a control circuit 207 energized by the voltage across the capacitor 208 for applying an on and off control signal to the gate terminal of the MOSFET 202, a resistor 213 connected between the gate terminal of the MOSFET 202 and the control circuit 207, and a voltage feedback transmission circuit 218a, 218b including a photocoupler for applying the voltage across the capacitor 205 to the control circuit 207.
In the step-down DC/DC converter of the above arrangement, depending on a change in the voltage across the capacitor 205, which is a low DC output voltage obtained through the voltage feedback transmission circuit 218a, 218b, the duration of the on and off control signal applied from the control circuit 207 to the gate terminal of the MOSFET 202 varies for thereby controlling the on period of the MOSFET 202 to stabilize the voltage across the capacitor 205, i.e., the voltage supplied to the load 206.
However, the conventional step-down DC/DC converters described above have the following problems:
(1) The conventional step-down DC/DC converters shown in FIGS. 1 and 3:
When the main transistor 102 is turned on, the emitter potential of the main transistor 102 is substantially equalized to the, potential of the positive terminal of the DC power supply 101. Therefore, in order to keep the main transistor 102 turned on, it is necessary to make the voltage of the on control signal applied from the control circuit 107 to the base terminal of the main transistor 102 substantially equal to the potential of the positive terminal of the DC power supply 101.
When the voltage of the DC power supply 101 is high, however, it is difficult to make the voltage of the on control signal applied from the control circuit 107 to the base terminal of the main transistor 102 substantially equal to the potential of the positive terminal of the DC power supply 101, resulting the possibility of in a failure to generate a low DC output voltage from the high DC input voltage.
(2) The conventional step-down DC/DC converter shown in FIG. 4:
Since the ground potential of the control circuit 207 which applies the on and off control signal to the gate terminal of the MOSFET 202 is equal to the source potential of the MOSFET 202, the on control signal applied from the control circuit 207 to the gate terminal of the MOSFET 202 in order to keep the MOSFET 202 turned on may be of a low voltage. Therefore, even when the voltage of the DC power supply 201 is high, it is possible to generate a low DC output voltage from the high DC input voltage.
However, because the power supply for energizing the control circuit 207 comprises an R.cndot.C (resistor and capacitor) charging circuit which comprises the resistor 212 having one terminal connected to the positive terminal and the capacitor 208 connected to the other terminal of the resistor 212, it is necessary to reduce the current consumed by the control circuit 207. Such a requirement places significant limitations on the main transistor used and its energizing circuit, the frequency of on and off cycles of the main transistor, etc.
Furthermore, common-mode noise tends to be introduced because the voltage feedback for controlling the output voltage is performed by the voltage feedback transmission circuit 218a, 218b including the photocoupler.